Accuracy control of a circuit line width required in a semiconductor device is increasingly required in recent years. Such a semiconductor device is manufactured by forming a circuit by exposing and transferring a pattern on a wafer by a reduced-projection exposure apparatus called a stepper by using an original pattern (called a photolithography mask or a reticle, and hereinafter collectively called a mask) in which a circuit pattern is formed. Therefore, a mask for transferring a fine circuit pattern on a wafer is manufactured by using a pattern drawing apparatus using an electron beam. The pattern drawing apparatus can draw a fine circuit pattern. The pattern circuit apparatus can directly draw a pattern circuit on a wafer.
Improvement of yield is essential in manufacturing of LSI, such as a central processing unit (CPU) and a field programmable gate array (FPGA), requiring a high manufacturing cost. One of significant factors to reduce yield is a pattern defect of a mask used to expose and transfer an ultra fine pattern on a semiconductor wafer by a photolithography technique. In recent years, as the size of an LSI pattern formed on a semiconductor wafer is miniaturized, a size to be detected as a pattern defect is significantly reduced. Therefore, a highly accurate pattern inspection apparatus to inspect a defect of a transfer mask used in LSI manufacturing is desirable.
An inspection method is known in which an optical image, in which a pattern formed on a sample of such as a photolithography mask is imaged at a predetermined magnification by using an expansion optical system is compared with design data or an optical image, in which the same pattern on the sample is imaged. Examples of a pattern inspection method include a “die-to-die inspection” and a “die-to-database inspection”. In the die-to-die inspection, data of optical images, in which the same patterns at different locations on the same mask are imaged, are compared each other. In the die-to-database inspection, drawing data (pattern data), in which pattern-designed CAD data is converted into an apparatus input format to be input by a drawing apparatus when a pattern is drawn to a mask, is input to an inspection apparatus, design image data (a reference image) is generated based on the drawing data, and the design image data and an optical image formed by the pattern and being measurement data are compared. In an inspection method in such an inspection apparatus, a sample is disposed on a stage and inspected by which a beam scans on the sample when the stage is moved. The sample is irradiated with the beam by a light source and a lighting optical system. A light transmitted through or reflected by the sample is imaged on a light detector via the optical system. An image formed by the light detector is sent to a comparison circuit as measurement data. In the comparison circuit, after images are positioned each other, measurement data and reference data are compared in accordance with an appropriate algorithm. In the case of nonconformity, it is determined that there is a defective pattern.
While a pattern is miniaturized, by using a mask as an original pattern, a lithography technique in which a circuit is formed by exposing and transferring a pattern on a wafer by a reduced-projection exposure apparatus is continuously used. Therefore, it becomes difficult to secure an allowable range with respect to a pattern defect of a mask and a variation allowable range (margin) of process conditions during exposure transfer to improve a manufacturing yield of a wafer. Conventionally, variation margins in the process conditions are absorbed mainly by strictly setting an allowable value of a form defect and increasing dimensional accuracy of a mask pattern form. However, in addition to that each figure included in a pattern is formed according to a designed size, high position accuracy of a pattern on a mask and line width dimensional accuracy of the pattern are required to the quality of a mask to be inspected.
Further, a defect correction method for a defect determined by a pattern inspection is diversified. Conventionally, the following methods are used as a correction method: a method in which a pattern is removed by spattering by a focused ion beam (FIB); a method in which a metal film is deposited by a laser chemical vapor deposition (laser CVD); and a method in which an amorphous carbon film is deposited as a pattern by the FIB or an electron beam. The Japanese Patent application Publication No. 2012-022323 discloses a method for correcting a transfer image of a photolithography mask by changing, by femtosecond laser pulse, a transmittance of a substrate including a pattern of the photolithography mask.
Therefore, an inspection apparatus and an inspection method are required which can highly accurately inspect a mask in which a defect has been corrected such that position accuracy and line width dimensional accuracy of the pattern of a photolithography mask are improved.